607 



facts would be most important in comparing the tables with an aim to 

 understanding such differences as are found in our results. He states 

 that in certain stages, presumably between 6 mm. and 8,95 mm. total 

 length, there is a reduction in the number of sex-cells due to de- 

 generation. His numbers, 225, 158, 196, 218 and 305 are, with the 

 exception of the last, far below the numbers that I found in corre- 

 sponding stages. In one of my specimens, 6,3 mm. total length, I counted 

 a total of only 302, but this was from a nest of which the other three 

 embryos likewise contained a relatively small number of sex-cells. The 

 average number found in my 17 specimens ranging through these stages, 

 was 1,110. Individual differences in the number of sex-cells, which I 

 have shown to be so marked, may have played a role, but it is difficult 

 to conceive that the laws of chance could play such a trick as to give 

 DusTiN a series of embryos with such low numbers of sex-cells as he 

 records. If any one doubts the care with which my counting was per- 

 formed, let him examine my paper and note the precautions taken and 

 the general character of the results. In a single specimen. No. 61 

 (8,95 mm. in length), Dustin finds 368 sex-cells, and claims that this 

 increase in number over those counts reported in his somewhat younger 

 stages as given above proves, when associated with the absence of mi- 

 totic figures, that sex-cells have arisen by transformation from peritoneal 

 cells. Even granting the reliability of his figures, which I am not willing 

 to do, such a difference could come well within the range of individual 

 variation. The extreme range of this is shown in my tables. Dustin 

 lays himself open to very severe criticism if he is willing to make such 

 fundamental conclusions upon the basis of counts of the sex- cells of but 

 one specimen, or of two specimens, if one could assume that his specimen 

 No. 60 with 305 sex-cells belonged to this stage. 



He speaks of a rapid degeneration of sex-cells that have not 

 reached the sex-gland anlagen. According to his account, this begins in 

 the 6 mm. stage and continues through the 8,95 mm. stage. I made 

 a careful study of this point when I was writing my papers on Chrys- 

 emys, and since then. It resulted in my finding but few sex-cells that 

 appeared as though they might be in process of degeneration. It is 

 difficult to make a precise count of these degenerating cells because 

 they grade insensibly into healthy sex-cells, but I am convinced that 

 very few sex-cells degenerate up to and including the 8 mm. stage. At 

 no period during embryonic development or adult life, is this sex-cell 

 degeneration sufficiently general to even remotely suggest the necessity 

 of a replacement by the differentiation of a new generation of sex-cells 

 from the peritoneal cells or from any other source. I hope that it may 

 not seem like mere retaliation for me to suggest that Dustin may have 

 mistaken ordinary entoderm cells in process of mitotic division for de- 

 generating sex-cells. Seriously, it seems quite possible that such might 

 have been the case. 



In any case, a consideration of the further development of the sex- 

 cells need deal only with those that have reached the sex-gland. 

 Dustin does not claim that sex-cells that have reached the sex-glands 

 are undergoing degeneration. 



